Various apparatus have been used for removing earth in a tunneling operation including the use of tunnel boring machines which are basically augers, to the use of digging machines such as back hoe type equipment. Tunnel boring machines commonly in use employ a rotating toothed cutting head at the end of a housing. As the cutting head rotates, the soil is loosened and passes into the housing where it is removed. At present there are two commonly employed techniques for removal of the debris from the tunneling operation, augers for dry spoil and slurries.
The auger systems use a central auger which rotates with the cutting head and moves the soil rearwardly and onto a conveyor for removal. Auger based tunneling machines often use a motor and gear train in the jacking pit to rotate the auger. Auger systems suffer drawbacks in some soil types. For example, loose or soft soil may increase in volume when exposed to air. Thus when drilling in loose soil, the soil may expand as it enters the cutting head and auger system and cause flooding of the housing. There have been machines developed which rely on the use of flood doors or gates to attempt to control the rate of soil transfer to maintain the pressure balance. Another problem is encountered in soil containing large rocks, where the rocks may be too large to enter the auger and may clog the removal of soil from the cutting head. While some rocks may be able to enter the auger system, there is still the possibility that they may become jammed in the auger system.
The other commonly employed method of removing soil involves the use of slurry systems. In this method the machine uses water to turn the excavated material into a pumpable fluid. The slurry normally requires a 15% solid mixture to achieve pumpable characteristics. Slurry-based machines have many drawbacks especially in colder climates where the slurry may be at risk of freezing during processing and disposal. In addition, silicifying materials such as clays take time and a lot of water to form into a slurry, whereas, sand courses absorb and disburse the water. In some types of soils the high pressure water may enlarge the diameter of the tunnel beyond the machine diameter and cause unexpected cave-ins. Since one must dispose of both a liquid and a solid waste, contaminated soils are an increasing environmental problem.
One particular group of tunnelling machines are those known as micro-tunnelling machines, which typically have diameters in the range of 2 meters or less, more particularly 1 to 1.5 meters in diameter. As these machines have very small diameters, they are generally remotely controlled from the jacking pit. These machines use a single motor and gear train in the jacking pit to rotate both the cutting head and auger simultaneously. Cutter head mining power loss on long drives is tremendous which is often the limiting factor in determining the length of the drive. Rotation of the cutting head with the auger also means that head rotation cannot be reversed as the auger will only move the spoil away from the face when rotated in one direction.